In the electronics industry, the tendency has been to reduce the size of electronic devices such as camcorders and portable telephones while increasing performance and speed. Integrated circuit packages for complex systems typically are comprised of multiple interconnected integrated circuit chips. The integrated circuit chips usually are made from a semiconductor material such as silicon or gallium arsenide. The integrated circuit chips may be mounted in packages that are then mounted on printed wiring boards.
Packages including integrated circuit chips typically have numerous external pins that are mechanically attached by solder or by a variety of other known techniques to conductor patterns on the printed wiring board.
Typically, the packages on which these integrated semiconductor chips, or semiconductor dies, are mounted include a substrate or other chip-mounting device. One example of such a substrate is a leadframe. Leadframes also typically include at least an area on which an integrated circuit chip is mounted and multiple power, ground, and/or signal leads to which power, ground, and/or signal sites of the integrated semiconductor die are electronically attached. The area on which the integrated circuit is mounted is typically called a die pad. The multiple leads typically form the outer frame of the leadframe. The die pad is typically connected to the outer frame leads by tiebars so that the whole leadframe is a single integral piece of metal.
Leadframes have been used extensively in the integrated circuit packaging industry mainly because of their low manufacturing cost and high reliability. Recent development such dual or multi-row leadframes have been proposed to increase packaging density and further reduce cost.
Typical leadframe packages include a die pad, surrounded by a number of leads. The leads are temporarily attached to the die pad by the tiebars. An integrated circuit chip is attached to the die pad using a conductive adhesive such as silver epoxy. Such an adhesive is typically called a die attach adhesive. The die attach adhesive is cured after the die attach. After the die is attached to the die pad, a wire-bonding process is used to make electrical interconnections between the integrated circuit and the leads of the leadframe. After wire bonding, the leadframe with the integrated circuit attached is encapsulated using a mold compound. Such enclosures may include encapsulant in a plastic, epoxy or a multi-part housing made of plastic ceramic, or metal with the purpose of protecting the leadframe and the attached chip from physical, electrical, and/or chemical damage. Finally, post mold curing and singulation steps are conducted to complete the packaging process.
In typical leadframe packages, the semiconductor die mounted is smaller than or of the same size of the die pad. In such a configuration, the surrounding leads occupy space where there is no functional semiconductor device. Therefore the density of semiconductor devices on the leadframe is limited. The current trend of the semiconductor industry is to increase the device density on the leadframe. Therefore such wasted space in the typical leadframe design presents a problem.
Certain methods are proposed to solve this problem. One is the overhang die approach. In this approach, the semiconductor die is positioned in such a way that the edge portion of the semiconductor die overhangs the leads of the leadframe. In so doing, the leadframe could be made smaller and the previously wasted space is utilized because that space is now occupied by the edge portion of the semiconductor die. However, the overhang approach has various problems. First, it has difficulty in die attach adhesive dispense for the overhung semiconductor die and the adhesive boundary is difficult to define. Second, because of the existence of the tiebar connecting the die pad and the leads, it is difficult to increase the lead count, especially for dual or multi-row leadframes.
Another approach is the chip on lead approach. In this approach, the die pad is eliminated from the leadframe and only the leads are left. Instead of being attached to the die pad, the edge portion of the semiconductor die is placed on top of the leads directly. In so doing, the leadframe could be made smaller and the previously wasted space is utilized because that space is now occupied by the edge portion of the semiconductor die. However, the chip on lead approach also has various problems. First, because the die pad is eliminated, there is a large contact area between the die attach adhesive and the mold compound underneath the semiconductor die. This area is known to have high risk of delamination. Second, the semiconductor die is directly in contact with the leads of the leadframe. Typically the tips of the leads are coated with silver. Silver tends to migrate through the die attach adhesive layer and reach the semiconductor die, resulting in leakage problems that manifest as field failure over time.
Thus, a need still remains for reducing the difficulty in die attach adhesive dispense, maximizing the lead counts of the leadframe, alleviating the delamination problem, and solving the silver migration problem for the overhang die and the chip on lead approaches. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations and the diminishing opportunities for meaningful product differentiation in the marketplace, it is critical that answers be found for these problems. Additionally, the need to reduce costs, improve efficiencies and performance, and meet competitive pressures, adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.